Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1265592
The present invention relates to a metallized
dielectric foil for making electrical capacitors
of the coiled type, as well as to capacitors obtained.
It is known that, to make electrical capacitors
5 of the coiled type, dielectric foils are used, consti-
tuted by a supple film-support made of paper or synthe-
tic material an~ by reinforcements, formed either
by a fine sheet of conducting metal or by deposition
on one of its faces, for example by vaporization,
10 of a fine layer of metal (zinc or other) adapted
to constitute an electrically conducting reinforcement.
Each foil presents along one of its longitudinal
edges a non-metallized strip or unco-~ered margin,
w~ilst, on the opposite edge, the layer of metal
15 presents, on the contrary, a considerable thickness.
To obtain a capacitor by coiling, two dielectric
foils of the above type are superposed, care being
takenl on the one hand, to inverse them so that the
thicker edge of the one is disposed above the margin
20Of the other, on the other hand, to introduce a slight
transverse shift between the two foils.
The cylindrical body made by winding such a
complex on itself is then provided with two frontal
cc,ntact terminals which are connected to one or the
25other of the two thick metallized edges, and the
whole is generally placed inside an insulating protec-
tive envelope which leaves the end terrninals uncovered.
The dielectric of such a capacitor inevitably
comprises faults, with the result that, upon first
30charging, this dielectric is the seat of localized
discharges, the arcs thus produced provoking volatiza-
tion of the metal around the faults. Insulation is
thus established, this phenomenon being known as
"auto-regeneration" or "cicatrization".
However, in the case of capacitors subjected
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to relatively high voltages, it may happen that the
zone to be regenerated or "cicatrized" covers a large
number of turns and that such regeneration becomes
impossible. It is to overcome this risk that the
metallized dielectric foils are provided with narrow
non-metallized separating strips which extend trans-
versely from the free edge or margin up to a short
distance from the thicker edge. Surfaces or compart-
ments are thus formed along the foil which are electri-
cally connected to one another only by narrow metal-
lized portions.
Despite this compartmentation, it may happen
that the energy 1~ CV2 of the discharge at the level
of the connecting portions is still too low to burn
these narrow portions and that the capacitor is really
damaged. It has thus been proposed to provide the
dielectric foils with non-metallized strips oriented
longitudinally and presenting a discontinuous profile,
in order to determine a series of meltable connecting
bridges adapted to limit the energy to a value capable
of allowing perfect regeneration or cicatrization.
The res~llts obtained are satisfactory from the
standpoint of the capacitor's behaviour, but, on
the contrary, production of the interrupted strips
raises difficult technical problems, which obviously
have an unfavourable effect on the cost price of
the finished articles.
It is an object of the present invention to
overcome this drawback, essentially by providing
the dielectric foils with a narrow zone with a very
reduced thickness of metal, said zone beinq oriented
longitudinally along the thick edge and being penetra-
ted by the end of the transverse compartmentation
! strips.
~ 35 Tests have shown that such a thinned longitudinal
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zone, easily obtainable during vaporization of the
metal on the film-support, behaved from the functional
standpoint at least as satisfactorily as the conventio-
nal discontinuous strips.
S The invention will be more readily understood
on reading the following description with reference
to the accompanying drawings, in which:
Fig. 1 shows very schematically in section a
metallized dielectric foil made in accordance with
10 the present invention.
Fig. 2 illustrates the obtaining of the complex
intended to be wound for making a capacitor.
Referring now to the drawings, reference 1 in
Fig. 1 designates a film-support of the type usually
15 used for making the dielectric foils, which film-
support 1, for example made of paper, is coated on
one of its faces with a metallized layer 2 obtained
by vaporization in vacuo or like deposit. This layer
2 stops in conventional manner at a short distance
20 from one of the longitudinal edges of the strip which
constitutes the film-support 1, consequently defining
a longitudinal, non-metallized strip or margin refe-
renced 3. Alon~3 the opposite longitudinal edge, the
metallized layer 2 presents, on the contrary, a conti-
25 nuous extra th.ickness 4. Finally, this layer 2 isinterrupted in conventional manner by transverse
strips 5 which, starting from the margin 3, stop
immediately level with extra thickness 4, thus deEining
on the sheet a series of separate compartments 6
30 which communicate with one another only at the level
of said extra thickness 4.
According to the present invention, the longitudi-
nal zone 7 at the level of which the extra thickness
4 is connected with each of the compartments 6 is
35 covered only by a very small,thickness of metal coa-
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ting, consequently defining in section a profile
substantially in the form of a dish. Of course, the
transverse strips 5 terminate inside this zone 7,
as is clearly shown in the drawing.
Tests have shown that it was easy to make zone
7 during the operation of metallization by vaporization
in vacuo and, by a precise calculation, each of the
parts of this zone 7 included between two adjacent
strips 5 could be given the desired resistivity.
The capacitor is made in conventional manner
by superposing two foils 8 and 9 (Fig. 2) disposed
so that the extra thick edge 4 of the one is placed
opposite the edge 4 of the other, and by introducing
a slight transverse offset between said foils 8 and
15 9. Tests have shown that the most advantageous results
were obtained by using for complex 8-9 a sheet 9
presenting a thin-layer zone 7 and a sheet 8 comprising
on]y the uncovered strip or margin 3 without strips
5 nor zone 7. This complex 8-9 is wound and treated
20 in conventional manner, and it has been observed
thclt, upon fixst charging of such a capacitor, the
metal which forms the layer covering zone 7 of the
sheet 9 vaporizes further to a possible puncture
occurring between the different strips 5. The strip
25 in which such puncture occurs is thus insulated and
the energy 1~ CV2 contained in the winding is not
released at the level oE the punctur~ with the risk
of destroying the capacitor.
It goes without saying that the complex may
30 be constituted with the aid of two correctly disposed
foils 9.
It must, moreover, be understood that the fore-
going description has been given only hy way of example
ancl that it in no way limits the domain of the inven-
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tion whi.ch would not be exceeded by replacing thedetails of execution described by any other equi-
valents. It goes without saying that the invention
also covers the capacitors made with the aid of the
improved dielectric foils of the type described herein-
above.
